Large CP violation in $\Lambda^0_b \to pK^-\pi^+\pi^-$ and its U-spin partner decays

The LHCb Collaboration has recently found a large CP-violating rate asymmetry in the $b$-baryon decay $\Lambda^0_b \to pK^-\pi^+\pi^-$. This is the first observation of CP violation in baryon processes, opening a new window to test its standard model origin. Many more baryon decays are expected to exhibit observable signals of CP violation. We show that there also exists large CP violation in the U-spin partner decay mode, $\Xi^0_b \to \Sigma^+\pi^- K^+K^-$, with rate asymmetry $$A_{CP}(\Xi^0_b \to \Sigma^+\pi^- K^+ K^-) = - A_{CP}(\Lambda^0_b \to p K^- \pi^+ \pi^-)~ \frac{Br(\Lambda^0_b \to p K^- \pi^+ \pi^-)}{Br(\Xi^0_b \to \Sigma^+ \pi^- K^+ K^-)}~ \frac{\tau^{\Xi_b}}{\tau^{\Lambda_b}} $$ in the U-spin symmetry limit. By neglecting a subleading contribution in the amplitudes, we obtain $$A_{CP}(\Lambda^0_b \to p \pi^+ \pi^- \pi^-) = A_{CP}( \Xi^0_b \to \Sigma^+\pi^- K^+ K^-) = -(12 \pm 3 ) \%. $$ These predictions provide crucial tests for the standard model.